This invention relates to apparatus using ultrasonic energy and particularly to apparatus for determining the rate of flow of a fluid in which the propagation times of ultrasonic signals transmitted through the fluid are detected to determine flow rate. Examples of such sensors are described in U.S. Pat. Nos. 6,178,827, 6,370,963 and 6,422,093, the disclosures of which are included herein by reference.
Ultrasonic transit time flow sensors, also known as xe2x80x9ctime of flight ultrasonic flow sensorsxe2x80x9d, detect the acoustic propagation time difference between ultrasonic signals transmitted upstream and downstream through a flowing fluid and process the time difference information to derive the fluid flow rate. The propagation time difference is very small and must be detected with high resolution and stability for the sensor to be practical. The ultrasonic transducers and their interfaces to the fluid are generally the major sources of detection instability. It is an object of this invention to minimize this problem in a cost effective way.
Prior art ultrasonic transit time flow sensors generally beam acoustic energy through only a small part of an overall flow profile and infer the overall fluid flow from that measurement. Because the fluid flow through the sensor is generally not uniform and because the flow distribution varies with flow rate and conditioning, determining volumetric flow rate is prone to error. At substantial additional cost, some of these sensors use internal reflective surfaces or multiple pairs of transducers to detect the flow rate at several locations in the flow profile. It is a further object of this invention to provide an ultrasonic time of flight flow sensor capable of measuring a flow rate of a large portion of the fluid at low cost.
The invention provides an ultrasonic transducer for a time of flight flow sensor that comprises a housing, a conformal layer, a thickness-mode piezoelectric transducer element and means for clamping the conformal layer between one of the faces of the transducer element and an internal surface portion of the housing. The preferred transducer also comprises an acoustic isolation means for acoustically isolating a second face of the piezoelectric element from the housing; and pre-loading means for biasing the piezoelectric element toward the internal surface portion of the housing so as to clamp the conformal layer therebetween.
Preferred embodiments of the present invention use a piezoelectric transducer element having both of its two electrical contacts on one surface which is brought into mechanical contact with a mating connector to form part of a stack of acoustic components. A quarter wave resonator and a massive acoustic load are preferably attached to the side of the connector opposite to the electrical contacts to complete the stack. The stack is spring-loaded so as to be squeezed against a lateral inside surface of its housing, where a compliant film is located to fill in the low spots, and to generally provide a conformable interface when clamped between the mating piezoelectric element and housing surfaces. A preferred spring is tensioned with a wedge which can be controllably inserted into the housing in order to adjust the spring bias so that the preload pressure is evenly distributed on the element. The entire assembly is then preferably encapsulated. Such an assembly exhibits excellent acoustic stability and uses only low cost components. The preferred combination of spring pre-loading and a compliant coupling film makes the assembly very forgiving of mechanical dimensional changes so that the acoustic coupling between element and housing is stable, thereby reducing transit time detection error.
These transducers are preferably spaced apart along the flow direction on opposite sides of a flow sensor body and communicate directly with each other through the fluid, preferably on a diagonal path. For smaller flow sensors, for example, with a 1xe2x80x3 bore, transducers of the invention may comprise single piezoelectric elements which may be relatively long and thin, for example, 0.6xe2x80x3 long and 0.125xe2x80x3 wide, in order to provide a relatively wide acoustic beam passing through a major fraction of the fluid that flows through the sensor. Such a long and thin element is practical because neither its assembly and mounting procedure, nor its subsequent pressure loading is likely to stress it to fracture. The use of an equivalent element and stack assembly in a conventionally configured sensor of the same size is generally impractical because the elements, being round and relatively large, and mounted at the end of the transducer, would require considerably more material, which would substantially increase the cost of the sensor.
For larger flow sensors, transducers of the invention preferably comprise two or more piezoelectric elements in order to effectively detect the fluid flow rate over most of the flow passage. The preferred piezoelectric elements may be in contact with a single mating connector which can be configured to connect them in parallel or to allow access to them individually. When individual element access is desired, the elements can be selected for operation at different times and their detected transit times weighted for location in the flow passage to enable very high volumetric detection precision to be obtained. Individual element access also enables a variety of element characteristics, such as different operating frequencies, to be accommodated as may be desired.
Although it is believed that the foregoing recital of features and advantages may be of use to one who is skilled in the art and wishes to learn how to practice the invention, it will be recognized that the foregoing is not intended to list all of the features and advantages. Moreover, it may be noted that various embodiments of the invention may provide various combinations of the hereinbefore cited features and advantages of the invention, and that less than all of the recited features and advantages of the invention may be provided by some embodiments.